1. Field of the Invention
The present invention relates to a sheet processing apparatus capable of flattening a folded-back portion of a saddle-stitched sheet bundle, and to an image forming apparatus having the sheet processing apparatus.
2. Description of Related Art
There is proposed a sheet processing apparatus which saddle-stitches a plurality of piled sheets on which images are formed by an image forming apparatus. There is proposed an apparatus capable of pressing a folded-back surface (stitched side) of a saddle-stitched sheet bundle with a roller, and capable of flattening the same as shown in FIG. 25 (Japanese Patent Application Laid-open No. 2004-345863).
In an image forming apparatus such as a copier, there is proposed an image forming system which produces sheets output from the image forming apparatus as a saddled-stitched and bound material by a sheet processing apparatus having a saddle-stitching and binding mechanism and a flattening mechanism (Japanese Patent Application Laid-open No. 2005-239414).
Further, in an image forming apparatus such as a copier, there is proposed a sheet processing apparatus which controls the flattening processing such that a protruding amount of a folded-back portion of a sheet bundle is changed in accordance with a thickness of the sheet bundle so that the folded back width is suitable for the thickness (Japanese Patent Application Laid-open No. 2006-290588).
In the saddle-stitching or center-folding binding operation, if a finished booklet is left as it is, the booklet is slightly opened and its appearance is deteriorated in some cases. To prevent this, there is conceived a structure for increasing a folding pressure at the time of center-folding operation.
However, when a folded-back surface of the center-folded and bound booklet is flattened to form a spine, if a center-folding pressure is too strong, a center-folded crease 980 remains on a spine as shown in FIG. 25 and a finished appearance of the spine is deteriorated in some cases. Especially when an image is formed on a spine, there is a possibility that the image is cracked along the crease and the finished appearance is deteriorated.
Depending upon whether the flattening processing should be performed, techniques required for the center-folding processing are completely different, but selections as to whether the flattening processing should be performed are varied depending upon a booklet style desired by a user.